One type of transmission known in the art in which a speed change ratio can be continuously varied, is a hydrostatic transmission (referred to hereafter as HST) comprising a variable capacity hydraulic pump and a variable capacity hydraulic motor.
This HST converts a motive force of an engine to a fluid flowrate and pressure by means of the variable capacity hydraulic pump, and transmits the output to the variable capacity hydraulic motor. The rotation of the motor can be controlled in a stepless fashion, including forward rotation, speed increase/decrease, backward rotation (i.e. rotation in a negative direction) and stop, by controlling the variable capacity hydraulic pump and the angle of a swash plate of the variable capacity hydraulic motor as shown in FIG. 7. Power is transmitted by a fluid flowing through an oil pressure pipe, so there is no mechanical restriction on the positions of the pump and motor, the disposition of these components relative to one another being a matter of free choice. For this reason, HST is widely used in construction machinery, for example.
However there is a problem with HST as regards power transmitting efficiency. As the power is transmitted by oil pressure, the transmitting efficiency is at most approximately 80%, and under normal partial load conditions, it is only of the order of 70-75%.
Further, to increase the speed of input rotation, the capacity of the variable capacity hydraulic motor must be decreased, but if the capacity is decreased, the oil pressure for the same load torque increases. In this case, the torque that can be used must also be controlled such that it decreases together with the rise of output rotation, and as a piston stroke has to be shortened to decrease the capacity, the efficiency also falls by a corresponding amount.
There is also a problem as regards the noise of the variable capacity hydraulic motor. The output rotation of HST is directly proportional to the rotation speed of the variable capacity hydraulic motor, i.e. to the piston speed, as shown in FIG. 8. Noise increases in direct proportion to the piston speed, consequently in the high rotation speed region noise increases, and more discomfort is caused by a rise of acoustic frequencies.
In addition to the aforesaid HST, there is also a mechanical-oil pressure transmission wherein transmission of the power is divided between a mechanical system and an oil pressure system, which is generally referred to as a hydromechanical transmission or HMT.
As one example of this, there is a combination of a differential gear device and HST which is used in driving aircraft generators, and an HMT is also proposed in Japanese Patent Publication (Tokkai Hei)1-250661.
According to this publication, power is transmitted not only by a fluid which has a large transmission loss, but also mechanically until certain conditions. The transmission efficiency is thereby increased. When the angle of the swash plate is in a neutral position, the piston stops, the rotation of the input axis and output axis coincide, and all the power is transmitted mechanically as shown in FIG. 10. The power transmission efficiency then reaches 95%. By controlling the angle of the swash plate, power of oil pressure is added, speed can be increased or decreased. Even when the output rotation speed is increased, a power transmission efficiency exceeding 90% is maintained and there is no variation of oil pressure for the same torque, which are major differences from an HST variable capacity hydraulic motor.
However, HMT has no functions to reverse the output rotation as shown in FIG. 9, and even when the rotation stops, the piston speed is a maximum in this stop position as shown in FIG. 10, so in practice it does not have a stable stop function.
Moreover, as it is required to mechanically transfer power from the input shaft to the output shaft, the positions of the input shaft and output shaft relative to one another are limited, and there is no design freedom as in the case of the input/output shafts of HST.
Further, the hydraulic pump on the input side and the hydraulic motor on the output side are connected not only mechanically but also hydrostatically. Consequently, it is not possible to make a connection to an external oil pressure line, and oil from the hydraulic pump cannot be used for other functions. In other words, as there is only one unique function, it is difficult to apply it to the case of construction machinery where it is required to share oil pressure lines.
HST permits free design, and vehicle wheels or the like may be directly fitted to final drive components. On the other hand, HMT has a low degree of freedom, is difficult to install in final drive components, and involves a more complex system than that of HST. For these reasons, the use of HMT is not so widespread.